1. Field of the Invention
This invention relates to a material for an organic electroluminescent device and an organic electroluminescent device fabricated using the material. More specifically, the present invention relates to an organic electroluminescent (which will hereinafter be abbreviated as "EL") material used for full color display and an organic EL device fabricated using the material.
2. Description of the Related Art
An organic EL device makes use of the phenomenon that holes injected from an anode and electrons injected from a cathode recombine in an emitter layer having a fluorescent capacity and radiates light at the time of deactivation from the excitation state. Researches on such a device have been started, paying attention to a high fluorescence quantum efficiency of an organic compound and its molecular structure permitting various designs. The luminescent brightness and efficiency of an organic compound was however insufficient for practical application. After that, Tang and Vanslyke reported that by the use of, instead of an emitter layer alone, a stacked structure in which the emitter layer is combined with a material having excellent hole transporting capacity (which will hereinafter be called "hole transport layer") brings about a drastic improvement in the performance of the device (Applied Physics Letter, 51, 913(1987)). With that report as an opportunity, the study has been concentrated to the technique of completely and functionally dividing the layer into a layer having a role of injecting holes (hole injection layer), a layer having a role of transporting electrons (electron transport layer) and the like. Owing to an improvement in the performance of each organic material in addition to the above-described progress in the study, an organic EL material is near industrialization as a display device (an emitter Layer, electron transport layer, hole transport layer and hole injection layer will hereinafter be called an "organic functional layer" collectively).
Recently, it has been reported that in the case of a green luminescent type, the use of a starburst amine for a hole injection layer brings about a brightness of at least 100 thousand cd/m.sup.2, a luminous efficiency of at least 101 m/W ("Monthly Display", September, 1995) and a half life of the brightness of at least 10 thousand hours when continuously driven. As an organic EL device assuming blue luminescence, it has been reported (in the special lecture at the 70th Spring Annual Meeting of The Chemical Society of Japan) that the use of a distyrylarylene derivative as a luminescent material brought about a brightness of at least 20 thousand cd/m.sup.2, a luminous efficiency of 51 m/W and a half life of at least 5000 hours. The study on a red luminescent material however has not yet reached a practically usable level, because an organic compound originally has a band gap wider than that of an inorganic semiconductor material so that molecular designing cannot be effected easily, a substance synthesized from it has difficulty in the film formation and an yield of purification conducted for purity increase is poor. Under such circumstances, as disclosed in Japanese Patent Laid-Open No. 152897/1991, an attempt has been made as a method to attain red or multi-color emission. In the method, a filter called "a color changing layer" is inserted in front of an organic EL device and this filter has an absorption in the luminescent wavelength from the organic EL device and at the same time emits fluorescence. At the time when a light emitted from the EL device passes through the filter, a portion of it changes in color and is taken out as red or multi-color luminescence. The above-described method is however accompanied with the drawbacks that since a quantum efficiency is limited because of a color change of a light emitted from EL through a filter, sufficient luminous efficiency cannot be attained and the use of a filter inevitably increases the cost. Disclosed examples of the EL material assuming red luminescence include (1) phthalocyanine compound, as described in Japanese Patent Laid-Open No. 288184/1995, represented by the following formula: ##STR2## wherein X represents a hydrogen atom; M is selected from the group consisting of magnesium, lithium, sodium, calcium, zinc, aluminum, gallium and indium; and in (Y).sub.B, B stands for 0 or 1 and when B stands for 1, Y is selected from the group consisting of chorine and bromine;
(2) a 4-hydroxyacridine compound, as described in Japanese Patent Laid-Open No. 166159/1995, represented by the following formula: ##STR3## wherein M stands for a metal in Groups II-III of the periodic table; (3) a violanthrone compound, as disclosed in Japanese Patent Laid-Open No. 90259/1995, which is represented by the following formula: ##STR4## wherein R.sub.1, R.sub.2, R.sub.3, R4, R.sub.5 and R6 each independently represents a hydrogen atom, a halogen atom, a lower alkyl group, a lower alkoxy group, a phenyl group, a dialkcylamirno group or a diphenylamino group. The structure of a red-emitting device fabricated using such a material will next be described with reference to FIG. 2. Over a transparent insulation supporting substrate (201), a transparent conductive thin-film (202) is stacked as an anode. Then, a hole transport layer (203), the above-described red-emitting layer (204) and an electron transport layer (205) are stacked successively over the thin film, followed by the formation of an upper cathode layer (206). The red-emitting layer (204) is poor in the injection and transporting capacities of holes and electrons so that an efficiency increase can be attained by sandwiching the red-emitting layer between the hole transport layer and electron transport layer. The above-described red-emitting material however cannot be put into practical use because its fluorescence quantum efficiency is low and it can emit light with a brightness of only about 1000 cd/m.sup.2 even if the amount of a current flowing through the device is increased.
Using examples of a coumarin derivative for attaining blue to white light emission for full-color display include (4) Japanese Patent Laid-Open No. 157815/1996, (5) Japanese Patent Laid-Open No. 126330/1995, and (6) a mixed material, as disclosed in Japanese Patent Laid-Open No. 188340/1995, of a coumarin derivative and a compound having a specific structure represented by the following formulas: ##STR5## wherein R.sub.1 to R.sub.5 each independently represents an atom or group selected from a hydrogen atom, a fluorine atom and alkyl, alkoxy, dialkylamino, alkanoyloxy, alkyloxycarbonyl, aryl, cyano, alkanoyl and trifluoromethyl groups, X represents O or NY, in which Y representing a hydrogen atom, an alkyl group or an aryl group, n stands for 0, 1 or 2 and R.sub.6 represents a hydrogen atom or a methyl group. When such an organic material is used as an emitter layer, stable blue light emission can be attained and furthermore, it becomes possible to emit a white light by mixing it with green- and red-dopant materials.
In the above-described prior art, the first problem resides in that in the case where the above coumarin derivative emits blue light and multi-color or red color emission is aimed at, another red-emitting material is necessary. In this case, the material for red light emission as described above in (1) Japanese Patent Laid-Open No. 288184/1995, (3) Japanese Patent Laid-Open No. 90259/1995 or (2) Japanese Patent Laid-Open No. 166159/1995; or the material which is described in (4) Japanese Patent Laid-Open No. 157815/1996 and has a structure represented by the following formula: ##STR6## is used.
Even if any one of the red-emitting materials is used, sufficient brightness cannot be attained so that an attempt for multicolor light emission inevitably lowers a color balance.
The second problem resides in the film forming property. The above-described coumarin derivative is a high molecule having a molecular weight of about 500,000 so that resistance heating type vacuum deposition, which is an ordinarily employed film formation method, cannot be employed for it. Instead, it is inevitable to adopt a spin coating method after dissolving the derivative in an organic solvent such as toluene or acetone. When the solution so obtained is coated at a rotational speed of 6000 rpm, the homogeneity of the thin film thus obtained is inferior by several % to several ten % to that of the film formed by the vacuum deposition method. Moreover, defects as large as several microns presumably occur in the film, which becomes a cause for shortening the life of the organic EL device.